Sica U.S. patent application Ser. No. 09/187,950, filed Nov. 6, 1998, and entitled "All-weather Flourescent Lamp With a Protective Assembly" (hereinafter "Sica"), discloses a protective assembly for a standard fluorescent lamp of the type having an elongated glass tube and a metal terminal cap at each end of the glass tube, each cap having a flange portion adjacent the glass tube. The assembly includes a protective tube that is preformed from a semi-rigid non-frangible polymeric transparent or translucent material that is stabilized against ultraviolet radiation and is received over the glass tube with its inner surface substantially uniformly spaced apart from the outer surface of the glass tube to form an air space for insulation of the lamp. The protective tube is substantially coextensive with the full diameter portion of the glass tube lengthwise of the lamp and is securely fastened to the flange portions of the lamp. The protective tube protects the lamp from impacts, thus reducing the possibility of the lamp being accidentally broken. In the event that the lamp does break, the protective tube remains intact and secured to the end caps of the lamp so as to form an enclosure that keeps broken glass fragments, phosphor powders and mercury from escaping to the environment.
The all-weather characteristic of the lamp of Sica is obtained by forming numerous small vent holes in portions of the protective tube proximate to the end caps. The vent holes are of a size such that fragments of glass from a broken lamp tube cannot pass through them. The number of vent holes is such that the air gap between the glass tube of the lamp and the protective tube is vented to permit the lamp to operate without substantial reduction in light output and without substantial discoloration of the protective tube in the range of ambient temperatures of from about 50.degree. F. to -30.degree. F. Sica is incorporated into the present specification for all purposes.
For proper operation of the lamp of Sica--i.e., substantially undiminished light output over the aforementioned broad range from moderately cool to very cold temperatures and prevention of discoloration and other effects of over-heating of the lamp near the upper end of the range--it is important that the vent holes be formed within a close tolerance. Variations in the diameters and lengths of the holes may significantly alter the ventilating properties, which can lead to either overcooling and loss of light output at low temperatures or overheating at temperatures near the high end of the desired operating range.